1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high-peak-power pulsed lasers used for the laser peening of solid materials and, more particularly, to methods and apparatus to control the application of the overlays applied for laser peening (also called laser shock peening, laser shock processing, and shock processing). The invention is especially useful for reducing the variability of the thickness of the overlays applied to the surface of the workpiece being laser shock peened.
2. Description of the Related Art
Old methods for shock processing of solid materials typically involve the use of high explosive materials or high-pressure gases, which are used to accelerate a plate, that strikes the solid material to produce shock waves therein. Such methods have several disadvantages. For example: (a) it is difficult and costly to shock process non-planar surfaces and complicated geometries, (b) storage and handling of the high explosive materials and high pressure gases pose a hazard, (c) the processes are difficult to automate and thus fail to meet some industrial needs and (d) high explosive materials and high pressure gases cannot be used in extreme environments such as high temperatures and high vacuum.
Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces. In shot peening, many small shot or beads are thrown at high speed against the surface of a material. The shot or beads sometimes escape from the treatment equipment and scatter in the surrounding area. Since the shot or beads might get into surrounding machinery and cause damage, shot peening usually cannot be used in a manufacturing line. Ordinarily such shot peening cannot be used on machined surfaces without a high likelihood of damaging them. In addition, shot peening has problems maintaining consistency of treatment caused by inherent wear of the shot by the shot peening equipment.
Laser peening equipment, however, can be incorporated into manufacturing lines without danger to surrounding equipment. Laser peening with coherent radiation has several advantages over what has been done before. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on preselected surface areas and the operating mode is easily changed. This allows flexibility in the desired shock (peening) pressure and careful control over the workpiece area to be laser peened. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to laser peen the workpiece repetitively. This is desirable where it is possible to enhance material properties in a stepwise fashion. Laser peening the workpiece several times at low pressures can avoid gross deformation, cracking, and spallation of the workpiece while nonplanar workpieces can be laser peened without the need of elaborate and costly shock focusing schemes.
Laser peening utilizes two overlays: a transparent overlay (usually water and hereinafter referred to as overlay water) and an energy absorbing opaque overlay; an oil based paint; an acrylic based black paint; or an energy-absorbing tape (and hereinafter referred to as paint). During processing, the paint is locally applied to the surface of the workpiece to be processed, followed by the application of the overlay water. A laser beam is directed to pass through the overlay water and is absorbed by the paint, causing a rapid vaporization of a thin layer of the paint and the generation of a high-amplitude shock wave in the workpiece. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue properties of the workpiece. A workpiece is typically processed by laser peening a matrix of overlapping spots that cover the fatigue critical zone of the workpiece.
The method of rapidly applying an energy absorbing overlay or paint, followed by a transparent overlay or overlay water, subsequently laser shock peening the workpiece, and then removing the overlay is presented in U.S. Pat. No. 5,471,559. This method has been successfully implemented into laser peening equipment and has dramatically decreased the processing time and cost; however, the variability in the thicknesses and/or uniformity of the paint or overlay water can potentially lead to differences in the resultant residual stresses in the workpiece and thus change the fatigue life capabilities of the laser-peened workpiece.
To provide a consistent laser shock peening process, the application of the paint and overlay water need to be monitored during processing and subsequently controlled to ensure that the thickness and/or uniformity of the paint and overlay water are the same for each laser shot. What is needed is a method to monitor and control the application of the paint and overlay water during processing to apply a consistent thickness and/or uniformity for each laser shot.